NASA has decided that its next launch vehicle for getting humans into space will be based on the space shuttle system, including its main engines, solid rocket boosters and external tank. There will be one big difference, though, instead of riding along the side of the new rocket, astronauts in the future will be riding on top on top of their next launcher -- above any debris that might fall off.

Speaking to reporters via telephone July 29, NASA Administrator Mike Griffin said the agency's plans are the result of an intensive Exploration Systems Architecture Study he ordered in late April to plot NASA's return to the moon by 2020. That study will be publicly unveiled in "a few weeks," Griffin said.

Sources with detailed knowledge of the study results said NASA will need to spend $5 billion to develop the crew launcher, a price tag that includes the solid rocket booster-based vehicle itself, a new upper stage and all necessary launch infrastructure.

The Crew Exploration Vehicle, which NASA hopes to field around 2011, is expected to cost another $5 billion to develop and would be designed both to service the space station and to carry astronauts to lunar orbit. A heavy-lift launcher capable of delivering 125 metric tons of cargo to low Earth orbit would be finished after the smaller crew launch vehicle, according to NASA's plan, and would also cost in the neighborhood of $5 billion to develop.

The Crew Exploration Vehicle, according to NASA's plan, will be a capsule capable of accommodating three people and a limited amount of cargo for space station missions, a crew of four for a lunar mission and up to six people to dock with an awaiting Mars-bound vehicle.

Unlike the shuttle orbiter, which is mounted to the side of the external tank and in the path of foam and other debris coming off the tank, the Crew Exploration Vehicle would launch at the top of the stack--out of the way of any debris the booster and upper stage might shed. NASA would adopt the same type of in-line design for the heavy-lift launcher as well, putting the cargo canister up top.

"As long as we put the crew and valuable cargo up above the tank we don't care what they shed," Griffin said, explaining to reporters why he remains confident in the shuttle-derived approach even after Discovery's close call.

Discovery's return to flight July 26 was marred by a close call with a falling chunk of insulating foam that broke free from its external tank about two minutes after liftoff. NASA officials previously had said they made great strides in reducing tank debris in the two and a half years since a chunk of foam brought down the Columbia, agency officials said. After the July 26 launch incident, they admitted they were wrong and said they clearly have more work to do and will not fly the shuttle again until the foam problem is solved.

Analysis of the spacecraft on orbit also revealed that two gap-fillers, ceramic fiber cloth used to keep the orbiter's heat-resistant tiles from clattering against one another during launch, had come loose and were jutting out of the spacecraft's underbelly. Discovery astronaut Stephen Robinson will attempt on Wednesday a spacewalk in an effort pluck out the filler material.

Prior to Discovery's return to flight, NASA also was closing in on its options for completing the International Space Station. Sources familiar with the planning effort said agency officials have been looking at two main scenarios.

One scenario involves conducting 17 shuttle flights before retiring the shuttle in September 2010, and includes launching Europe's and Japan's space station modules, but not outfitting them with additional science equipment. The other scenario involves conducting 11 shuttle flights and postponing launching the international partners' modules until NASA completes its proposed heavy-lift launcher. Knowledgeable sources said the 17-flight scenario will become more tentative with each month the shuttle is down and more or less untenable if the shuttle is not flying again come spring.

While Griffin said NASA has not given up on flying in September, experts outside the agency said it is unlikely the shuttle will be cleared for flight in time for that launch window.

"September seems awfully close," said James Hallock, a member of the Columbia Accident Investigation Board. "I think they've got a lot of things to do."

NASA would have one last shot for the year in November, but that window is only a couple days long. NASA first attempted to launch Discovery July 13, but that launch campaign was stood down nearly two weeks to give the agency time to troubleshoot a not yet fully resolved fuel sensor glitch.

One former spaceflight official said NASA could be facing a longer delay than it is ready to admit.

Griffin said he is not ready to concede defeat and has appointed a "tiger team" to tackle the foam problem and find a fix.

NASA was widely praised for swiftly deciding to stand down the shuttle fleet until it understands and fixes the foam problem. White House spokesman Scott McClellan said President George W. Bush is confident in Griffin and his team at NASA and appreciates the agency's commitment to safety. House Science Committee Chairman Sherwood Boehlert (R-N.Y.) and space and aeronautics subcommittee chairman Rep. Ken Calvert (R-Calif.) issued a joint statement saying "NASA is handling this situation exactly as it should." Former NASA Administrator Sean O'Keefe told Space News that NASA's recent actions are proof positive that a safety-oriented mindset has taken hold since the Columbia accident. "If there was still any doubt that the culture has changed, this should end it," O'Keefe said.

A number of space policy observers looking beyond the immediate engineering challenge ahead to the bigger picture, saw the turn of events as a significant setback for NASA.

"We have a series of commitments that require flying the shuttle, that's why this is such a nasty problem with no clear escape route," said John Logsdon, a Columbia Accident Investigation Board member who directs the Space Policy Institute at George Washington University here. "I don't envy the people who have to figure their way out of this corner."